Grapnels with articulated fingers have been used for a considerable time for such manipulations. By way of example, reference may be made to French Patent No. 1,528,762.
It sometimes happens that it is impossible to manoeuvre the grapnel. This impossibility can arise, e.g., from electrical or mechanical failure of the system actuating the articulated fingers, jamming of the fingers, or interfering objects preventing the movement of the fingers.
Whatever the cause of this inability to operate the articulated fingers of the grapnel, rapid action must be taken to disengage the fuel element from the grapnel in order not to arrest the nuclear reactor for too long. This task is all the more difficult as the intervention must be performed at a distance and the grapnel is under water.
Manipulating devices for fuel elements are known using a grapnel with articulated fingers, which have an integrated repair system enabling action to be taken on the locking of the grapnel in an emergency. However, these devices are fairly complex handling machines wherein the repair system is integrated with a mast and with a rigid framework (reference may be made, for example, to applicants French Patent Application No. 76-12545, filed Apr. 28, 1976). In addition to these systems only constitute emergency action on the actuation of the locking system and do not act directly on the fingers of the grapnel.